The present invention relates to a method of processing a workpiece and in particular, but not exclusively, to processing a semiconductor wafer.
In our European Patent Application No. 92306433.8 (0516344) we described a method of processing a workpiece having a multiplicity of recesses formed in an exposed surface, the method comprising depositing a first layer of material on the exposed surface until the first layer extends over all the recesses to close completely the openings of all the recesses in the exposed surface and subjecting the wafer and the first layer to elevated pressure and an elevated temperature sufficient to cause parts of the first layer to deform, without melting, to fill the respective recesses. This method is particularly suitable for forming interconnects in semi-conductor wafers and is most conveniently performed at elevated temperatures which are significantly below melting point, but which are yet high enough to reduce the yield strength of the first layer material to allow deformation at attainable pressures.
This method works well with aluminium and aluminium alloys, but it obviously impacts on the thermal budget available during the processing of the semi-conductor wafer. One of the effects of exceeding the thermal budget is to increase the risk of penetration of the silicon surface protecting barrier layer, by the interconnect material. This phenomenon, known as spiking, destroys the delicate active silicon region. GB-A-2247781 describes attempts to use laser heating to achieve reflow of metallised layers. It reveals significant energy window constraints due to optical ablation; the need for significant substrate heating and a requirement for anti-reflective coatings of refractory materials.
WO-A-95/22170 describes apparatus for providing a pressure pulse which may be accompanied by a pulse of thermal energy using a heating coil which heats the whole wafer.
These thermal budget considerations become even more significant as manufacturers move towards using copper as the interconnect material. The melting point of copper is much higher and its appropriate temperature at which the yield strength is suitable at attainable pressures is correspondingly higher.
The present invention consists in a method of processing a workpiece having a multiplicity of recesses formed in an exposed surface, the method comprising depositing a first layer of material on the exposed surface until the first layer extends over all the recesses to close completely the openings of all the recesses in the exposed surface and subjecting the wafer and the first layer to elevated pressure and elevated temperature sufficient to cause parts of the first layer to deform, without melting, to fill respective recesses characterised in that the heat is supplied in a thermal pulse within or to the first layer and the pressure is supplied during or throughout the thermal pulse.
Preferably the elevated pressure, which typically may be between 40-1000 bar (typically 700-750 bar) completely overlaps with the thermal pulse, indeed conveniently the elevated pressure is reached prior to the application of the thermal pulse, because of the difficulties which may occur with rapid pulsing of the pressure. The thermal pulse may be applied to the exposed surface region by region causing progressive filling of the recesses. For example it may be scanned over the surface using a raster scan or other scan protocol.
The thermal pulse may be applied using electro-magnetic radiation. Preferably this radiation is light and the light may be supplied from a coherent or incoherent sources, e.g. a laser, Xenon or Halogen source. The pulse may last up to 1 microsecond in length and a pulse length of the order of 100 nanoseconds is convenient. When a laser or concentrated light source is used consideration needs to be given to optical ablation. However, by using the laser or the like in a high pressure environment, the Applicant is able to broaden significantly the usuable energy window, as ablation will not occur until higher temperatures.
The energy of the pulse may be between 0.5 and 10 Joules per cm2 when the pulse length is of the order of 10""s or 100""s of nanoseconds, because a significant power will then result. A pulse of 1 joule per cm for 100 nanoseconds, for example, should produce sufficient power to heat a copper layer of xcexcm""s thickness. A man skilled in the art can identify the proper combination of energy and time parameters for any particular layer without undue experimentation.
Where the thermal pulse is supplied by radiation, the method may further comprise coating the exposed surface with a coating, which is absorbent at the radiation wavelength or wavelengths.
Thus when a layer is aluminum or aluminium alloy, an anti-reflective coating may be applied prior to the application of the radiation. A particular advantage of the Applicants"" process, vis-a-vis aluminium, is that the lower heat requirements should enable TiN to be used as an anti-reflective coating. As this is a preferred harrier layer material, this significantly simplifies apparatus requirements. In the case of copper, because it is inherently absorbent in the green, it should usually be possible to select a light source with an appropriate wave length or range of wave lengths.
Heating of the silicon substrate can be used to reduce the power requirements of the thermal pulse source, but often this will be undesirable, because some of the thermal budget advantages will be lost. In any case it will normally be desired to keep the substrate temperature significantly below 400xc2x0 C. and temperatures below around 350xc2x0 C. are desirable because heat induced damage to dielectric layers particularly low K dielectric layers at these temperatures is significantly at the temperatures.
Another particular advantage of keeping the substrate at as near room temperature as possible is that as the layer deforms to fill the recesses it comes into contact with barrier layers that are significantly cooler than the layer as it collapses. The barrier layer is therefore less likely to be spiked. Quenching of the layer as it contacts the significantly cooler under layers may also take place.
Although the invention has been defined above it is to be understood it includes any inventive combination of the features set out above or in the following description.